Characteristically plasticized polyvinyl chloride compositions, when in the form of molded articles, have relatively low toughness and crack propagation resistance at low temperatures.
In recent years, rigid polyblends of polyvinyl chloride (PVC) with rubbers and grafted rubbers have been developed to improve the toughness of PVC compounds. Rigid PVC polyblends with rubbers grafted with styrene and acrylonitrile were disclosed by Hayes in U.S. Pat. No. 2,802,809, granted Aug. 13, 1957. More sophisticated grafted rubbers have been developed for rigid PVC polyblends to improve their transparency for rigid vinyl bottles as disclosed by Beer in U.S. Pat. No. 3,636,138 granted July 18, 1972.
Currently, there is a great need for molded shoe parts, in particular, the soles and heels to provide shoe components that are economically manufactured and fabricated into shoes. A recent development has been the Unit-Sole or unitary sole and heel injection molded in the shape of a sole and heel. Such Unit Soles are combined with molded or fabricated uppers by adhesives to provide an attractive article of footwear at low fabricating costs. Unitary sole-heel articles have been molded of plasticized vinyl compounds but such moldings have relatively low flex crack endurance and low coefficients of friction for shoe soles. Block copolymers of butadiene and styrene have been used extensively but have relatively poor moldability and pliability and dimensional stability along with poor adhesion having to be chlorinated to establish adequate bonds. Polyurethane moldings have been developed that have high flex crack endurance but slow molding rates.
There has now been discovered a new and very useful plasticized polyvinyl chloride polyblend composition that can readily be injection molded into articles that are tough and resilient yet have high flex crack endurance over a wide temperature range. Such articles can be unitary shoe-heel moldings or other articles requiring tough, resilient, rubbery properties such as bumper guards and other impact absorbing elements.
These unusual flex-crack properties are gained without substantially affecting various other physical properties of the polyblend such as tensile strength, elongation to fail, impact strength and the like as compared to the known prior art materials cited and in particular plasticized vinyls or semi-rigid vinyl. The polyblends provide a superior coefficient of friction, low mold shrinkage, good adhesion, greater flex crack resistance at low temperatures and excellent moldability. Within the properties needed the composition can be varied providing a range of properties such as hardness and resiliency that allows the composition to be tailored to exacting uses.